A problem with known seat belt retractor pretensioning devices of the pyrotechnically activated-type is that the pressure conditions during the pretensioning operation vary considerably. These systems use a pressure tube with a piston which drives a series of metal balls or a ball-chain arrangement into a sprocket which drives the retractor webbing spool. A pyrotechnic pressure generator can be activated to produce the gas pressure needed to drive the piston and produce the pretensioning rotation. During the activation sequence, very high pressure peaks can damage parts of the pretensioning device, or disturb the motion sequences of the pretensioner device. Furthermore, the problem is that, unless the safety belt is provided with a force limiting device, the belt force at the start of the force-limited belt extraction motion following the pretensioning operation increases for a short time to a force limiting level that is higher than that defined by the force limiting device due to the pressure still prevailing in the pretensioning device. This effect is generally designated as a force limiting disturbance. In order to prevent these disadvantages resulting from excessive pressure, different solutions have been devised to prevent pressure peaks and reduce the pressure following the pretensioning action of the safety belt as quickly as possible.
It is the object of the invention to provide a cost-effective and functionally reliable pretensioning device with a high tensioning performance in which excessive gas pressure is reduced in a simple manner.